Lasers emitting ultrashort laser pulses having pulse durations of few tenths of femtoseconds are widely used for scientific research and technical applications. In all cases, it can be vitally important to measure the temporal profile of the laser pulse.
Within the context of this specification, characterizing an ultrashort laser means characterizing the temporal-spectral properties of the laser pulse.
Traditionally, ultrashort laser pulses have been characterized by non-linear autocorrelation diagnostics. Although relatively simple to implement, this analysis fails to provide complete information about the pulse. An improved approach is frequency resolved optical gating (FROG). With FROG it is possible to measure pulses over a wide range of wavelength and pulse duration. There are many versions of FROG, which in principle all rely on spectrally resolving a time-gated signal. A variety of different FROG measurement techniques is described in: Trebino, R.: Measuring Ultrashort Laser Pulses in the Time-Frequency Domain using Frequency-Resolved Optical Gating, Rev. Sci. Instrum. 68 (9), September 1997. In a FROG measurement, by spectrally resolving an autocorrelation signal, a two dimensional trace is created from which a characterization of a given pulse can be derived using an iterative algorithm.
Another approach, which is widely used for characterization of ultrashort laser pulses, is spectral phase interferometry for direct electric-field reconstruction (SPIDER). The SPIDER methods do not rely on temporal gating, but on interferometry in the spectral domain. The spectrum of a given pulse is made to interfere with a frequency-shifted (sheared) replica of itself. However, the quality of the phase measurement strongly depends on the accuracy of the delay between the two replicas and therefore, SPIDER-methods have high demands with respect to calibration of the optical setup.
A further approach with respect to the characterization of ultrashort laser pulses is known from: Miranda, M.: Simultaneous Compression and Characterization of Ultrashort Laser Pulses using Chirped Mirrors and Glass Wedges, Optics Express, 20(1), 688-697 (2012). The method, which is described in this scientific paper, applies a chirped mirror compressor setup to ensure that the pulse is negatively chirped. Subsequently, the pulse transits a pair of oppositely aligned glass wedges adding a certain amount of dispersion to the pulse. A second harmonic of the pulse is generated and the spectrum of said second harmonic is subsequently measured in the frequency domain.
For characterization of the pulse, a thickness of the glass, which is inserted in the beam path, is varied by shifting the glass wedges relative to each other. This is to vary the dispersion of the laser pulse. A plurality of second harmonic spectra is recorded as a function of the inserted glass thickness. In an iterative process, an error between the measured second harmonic spectra (for various glass thicknesses) and the corresponding simulations is minimized.
However, the various methods for characterizing an ultrashort laser pulse have various technical drawbacks and limitations such as a high degree of complexity, low sensitivity, the requirement for multiple laser pulses to perform the measurement, ambiguities in the measured profiles and high demands with respect to calibration of the optical setup, etc.